Geogrid has a certain influence on the strength and deformation characteristics of coarse-grained materials, and accurately describing the macroscopic mechanical behavior of geogrid-reinforced coarse-grained materials during loading is of great significance. In order to investigate the constitutive model which can reasonably describe its strength and deformation characteristics, based on the conventional triaxial test results of geogrid-reinforced coarse-grained materials, a nonlinear dilatancy equation is proposed in this study to reasonably describe the dilatancy and shrinkage characteristics of geogrid-reinforced coarse-grained materials. Compared with the Rowe and Cambridge model shear dilatancy equations, it is proved that this equation has advantages in predicting the shear dilatancy characteristics of geogrid-reinforced coarse-grained materials. Combined with the theory of generalized plasticity model, an elastoplastic constitutive model that can reflect the deformation and strength characteristics of geogrid-reinforced coarse-grained materials is proposed. There are totally 6 parameters in the model, all of which have clear physical meanings and can be determined by conventional triaxial tests. Finally, the accuracy of this model is fully proved by simulating the static tests under different confining pressures. The results show that the established model can reasonably describe the complex softening, hardening, and dilatancy and shrinkage behaviors of different types of geogrid-reinforced coarse-grained materials, providing an effective constitutive tool for corresponding engineering applications.